This invention concerns the atmospheric and subatmospheric treatment of particulate materials by a plasma-activated gas species to modify the particle surfaces. U.S. Pat. No. 4,478,643 discloses a pigment treated with a low temperature plasma wherein the pigment is maintained in a commercially available plasma asher for a time that is excessive and will cause pigment degradation. U.S. Pat. No. 5,234,723 discloses a process for treating particulates with a plasma-activated species at subatmospheric pressure by dropping a particulate through said treatment zone homogeneously. U.S. Pat. No. 5,176,938 discloses a process for treating particulates by passing them through a plasma flame. U.S. Pat. No. 5,340,618 discloses a method of treating a powder with an atmospheric pressure plasma by dispersing the powder within the reaction chamber using what appears to be a fluidized bed arrangement without addressing the problem of deagglomeration.
The process of the instant invention avoids the harsh treatment conditions of the '643 and '938 processes and the agglomeration of the '723 and '618 processes.
The organic particles contemplated for surface modification by the method of this invention have varying affinities for clumping together as agglomerates and/or aggregates by virtue of physical and/or chemical attractions among particles. It is believed that plasma activated species essentially will only interact with, and thereby treat, the surfaces of the clumps. When clumps or agglomerates are treated, only primary particles that are at or near the surface of the clump will be treated by the plasma activated species. It therefore follows that the larger the size of the clump, the smaller the proportion of the primary particles within the clump that can be treated by the gas species and hence the less effective the treatment process will be.
There is almost nothing in the prior art to guide one in the successful implementation of a modification process for materials that readily agglomerate. In this regard, it has now been discovered that successful surface modification of particles requires an integrated process for deagglomerating clumps of particles and treating them while in their deagglomerated state and before they have an opportunity to reagglomerate. It has been found necessary, for readily clumping particulate materials, to treat the particles while they are in a subdivided state, in order to achieve effective treatment at commercial production rates. The degree to which the clumps are subdivided must be no greater than a certain multiple of the mean dimension of the primary particles. The required conditions for the successful modification of normally clumping particulate organic materials by the process of this invention are as described in more detail hereafter.